In a multi-end DC (DC) power transmission system, a high-voltage DC circuit breaker is one of the most important devices. In the multi-end DC power transmission system, because a voltage level is high and the line resistance is low, once a line short-circuit fault occurs, a DC power transmission network and an alternating-current (AC) network are affected soon, and the fault must be cleared quickly. Therefore, the high-voltage DC circuit breaker needs to act fast and be able to reduce the fault duration to the greatest extent or suppress a faulty current, thereby reducing an attack of the fault on the AC/DC power transmission network. Because the high-voltage DC circuit breaker is connected in series to the power transmission line, in addition to that the circuit breaker is required to have a relatively high speed of switching on/off a circuit, the circuit breaker is required to have a loss as less as possible. A direction of a current in the high-voltage DC power transmission line is not determined, and the current may have two directions. Therefore, a circuit breaker is generally required to be able to distinguish DC current in two directions.
In the Chinese patent application CN102780200A, a conventional high-voltage DC circuit breaker is used to break a DC current, and a structure of the conventional high-voltage DC circuit breaker is constituted by three parts: an AC circuit breaker, an LC oscillation circuit, and an energy consumption element. After being opened, the AC circuit breaker generates an electric arc, the voltage of the electric arc resonates in the LC oscillation circuit, and when an oscillating current peak value reaches a magnitude of the DC current, the oscillating current can counterbalance the DC current, so that a zero crossing occurs at a port of the circuit breaker, so as to help extinguish the electric arc, thereby achieving the objective of switching off the DC current. Such a breaking manner may break a current in two directions and has an excessively small loss in normal working. However, an are extinguishing time of a conventional high-voltage DC circuit breaker is relatively long, which is about tens of milliseconds, so that a requirement of quickly isolating a fault of a multi-end DC power transmission system cannot be satisfied.
In the European patent EP0867998B1, a solid-state circuit breaker structure based on a semiconductor device is proposed and can be constituted by a switchable semiconductor device group and an energy consumption element. The switchable semiconductor device group is constituted by multiple low-voltage switchable semiconductor elements, and because a breaking speed of the switchable semiconductor device is extremely high, which is microsecond-scaled, a DC faulty current can be quickly switched off in this manner. However, because an on-state voltage drop of a semiconductor device group is great, a power transmission loss is increased, and power transmission efficiency is lowered.
In order to satisfy requirements of quickly isolating a DC faulty current and maintaining relatively high power transmission efficiency, the Chinese patent application CN102687221A discloses an apparatus and a method for breaking an electrical current of a power transmission or distribution line and a current limiting arrangement. A main circuit breaker, a high-speed switch, an auxiliary circuit breaker, and a non-linear resistor energy consumption element are included. In normal working mode, a line current flows through an auxiliary circuit and has a small on-state loss; and in faulty mode, the current is commutated to the main circuit breaker, and finally, the energy consumption element absorbs a breaking capability.
After a high-voltage DC circuit-breaking apparatus switches off a faulty current, the main circuit breaker withstands the voltage of several hundred kV, and the number of power semiconductor devices connected in series in one current direction can easily reach several hundreds. Because the power semiconductor device can only be conducted in a single direction, in order to switch off a faulty current in two current directions, a basic series-connection unit in the main circuit breaker in the high-voltage DC circuit breaking apparatus uses an anti-parallel or anti-series connection structure of two power semiconductor devices, and a number of power semiconductor devices in the main circuit breaker is doubled. During breaking in a first current direction, power semiconductor devices in a second current direction do not produce a beneficial effect on breaking the current or withstanding the voltage, which is equivalent to that a utilization ratio of the power semiconductor devices of the main circuit breaker is only 50%. Because costs of the power semiconductor devices occupy a large proportion of the total costs of the apparatus, in order to implement a function of breaking a current in two directions, costs of the apparatus are increased considerably.
Not only the increase of the power semiconductor devices in the second current direction in the main circuit breaker do not produce a beneficial effect, but also the power semiconductor devices in the second current direction are subject to the disadvantageous influence of overvoltage and overcurrent generated when the breaking occurs in the first direction. If the power semiconductor devices in the second current direction and the power semiconductor devices in the first current direction are in anti-parallel connection, when the breaking occurs in the first current direction, overvoltage is applied to the power semiconductor devices in the second current direction, and this voltage is a reverse voltage to the power semiconductor devices in the second current direction and would cause damage to the devices; and if the power semiconductor devices with an anti-parallel diode in the second current direction and the power semiconductor devices with an anti-parallel diode in the first current direction are connected in series in opposite directions, an excessively high abrupt current generated in the breaking process in the first current direction would flow through a freewheeling diode in the power semiconductor devices in the second current direction, which also exerts disadvantageous influence on the service life of the device.
The increased power semiconductor devices in the second current direction would also exert disadvantageous influence on the structural design and electrical design, and the power semiconductor devices in the first current direction have a consistent arrangement direction, so that the electric design and the structural design have consistency. The increase of the power semiconductor devices in the second current direction ruins the consistency in the original arrangement direction, resulting in increased difficulty in device layout, mounting, and wiring.